This application claims the benefit of Korean Patent Application No. 00-25767, filed May 15, 2000, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a lithium battery, and more particularly, to a lithium battery having improved safety and reliability by using a gel-state electrolyte solution.
2. Description of the Related Art
A lithium secondary battery generates electricity by lithium ions reciprocating between a cathode and an anode. The lithium secondary battery has a high energy density relative to the unit volume and voltage thereof compared to a Ni-Cd battery or a Ni-H battery, and the weight thereof is approximately half that of the Ni-Cd battery or the Ni-H battery. Thus, the lithium secondary battery is suitably used for small size, lightweight, longlasting electronic devices. As described above, lithium secondary batteries have attracted particular attention because of their excellent characteristics, such as high operating voltage, much better charging/discharging cycle and environmentally benign characteristics and so on, compared to conventional Ni-Cd batteries or Ni-H batteries.
However, since they are highly explosive, safety is a critical issue for practical use of lithium secondary batteries.
Lithium secondary batteries are classified according to the kind of electrolyte used, including lithium ion batteries using liquid electrolyte and lithium ion polymer batteries using polymer solid electrolyte.
The lithium ion battery generally utilizes a cylindrical case or a rectangular case as a case for hermetically sealing the electrode assembly. However, recently, a greater attention has been paid to a method in which a pouch is used instead of the case because use of the pouch increases energy density per unit weight or volume and allows attainment of small, lightweight batteries at low cost.
FIG. 1 is an exploded perspective view of an exemplary lithium ion battery using a pouch as a case.
Referring to FIG. 1, a lithium ion battery includes an electrode assembly 10 consisting of a cathode 11, an anode 12, a separator 13, and a case 20 for hermetically sealing the electrode assembly 10. Here, the electrode assembly 10 is formed by interposing the separator 13 between the cathode 11 and the anode 12 and winding the structure. A cathode tap 11xe2x80x2 and an anode tap 12xe2x80x2 serving as electrical paths between the electrode assembly 10 and the exterior, are drawn from the cathode 11 and the anode 12 to form electrode terminals 14 and 14xe2x80x2.
FIG. 2 is an exploded perspective view illustrating an exemplary conventional lithium ion polymer battery.
Referring to FIG. 2, a lithium ion polymer battery includes an electrode assembly 21 having a cathode, an anode and a separator, and a case 22 for hermetically sealing the electrode assembly 21. Electrode terminals (or lead wires) 24 and 24xe2x80x2 serving as electrical paths for inducing the current formed at the electrode assembly 21 to the exterior are installed to be connected to a cathode tap 23 and an anode tap 23xe2x80x2 to then be exposed by a predetermined length outside the case 22.
As described above, in the lithium ion battery shown in FIG. 1 and the lithium ion polymer battery shown in FIG. 2, the electrode assemblies 10 and 21 are put into the cases 20 and 22, respectively, and an electrolyte solution is inserted thereinto, with only parts of the electrode terminals 14 and 14xe2x80x2 and 24 and 24xe2x80x2, respectively, being exposed to the exterior. Then, heat and pressure are applied to each resultant structure so that thermally adhesive materials at the edges of the upper and lower case parts cause the upper and lower case parts to adhere together to then be hermetically sealed, thereby completing the batteries.
As described above, the electrolyte solution is injected in the latter part of the processing steps. Thus, in the case of using an organic solvent having a low boiling point, the electrode assemblies or pouches may be swollen, which deteriorates the battery reliability and safety.
To solve the problem, there have been proposed several methods, in which an electrode and an electrolyte are formed by hardening using ultraviolet rays or electron beams, or a gel-state electrolyte solution is coated on electrode plates without separate injection of an electrolyte solution, as disclosed in U.S. Pat. Nos. 5,972,539, 5,279,910, 5,437,942 and 5,340,368. In practical application of the proposed methods, swelling of an electrode assembly or a pouch for example, due to evaporation of the organic solvent, is somewhat mitigated, but is still not satisfactory.
Accordingly, it is an object of the present invention to provide a gel-state electrolyte solution which can effectively suppress swelling due to an electrolyte solution, and a lithium battery which can prevent the reliability and safety of the battery from lowering due to swelling of the electrolyte solution.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing objects of the present invention are achieved by providing a gel-state electrolyte solution consisting of a crosslinking product of prepolymer for forming epoxy resin and amine, a lithium salt and an organic solvent.
The foregoing objects of the present invention may also be achieved by providing a lithium battery including an electrode assembly consisting of a cathode, an anode and a separator interposed between the cathode and the anode, a gel-state electrolyte solution consisting of a crosslinking product of prepolymer for forming epoxy resin and amine, a lithium salt and an organic solvent, and a case for accommodating the electrode assembly and the electrolyte solution.
The electrolyte solution may be obtained by mixing a mixture of prepolymer for forming epoxy resin and amine with a mixture of the lithium salt and the organic solvent, injecting the resultant mixture into the case having the electrode assembly, and thermally polymerizing the same.
In particular, the electrode assembly may be a winding type electrode assembly, and the case is preferably in the form of a pouch. Thus, the energy density of the battery per unit weight and volume is increased. Also, attainment of a small size and light weight battery is allowed. Further, the cost of a raw material for a case can be reduced.